This invention relates to ohmic contacts and, more particularly, to ohmic contacts that are fabricated in III-V semiconductor materials where III and V refer to groups in the periodic table of elements.
Ohmic contacts are contacts which exhibit a linear current versus voltage characteristic. These type contacts are an important element of all semiconductor devices, such as field-effect transistors, light-emitting diodes and lasers. Ohmic contacts eliminate the inherently strong influence of the highly resistive surface depletion region present in the current-voltage characteristic of a metal-semiconductor junction.
One of the most common ohmic contacts utilized in the prior art is formed by alloying a metal to a semiconductor surface. The metal impurities are caused to act as donors or acceptors in the semiconductor material. Recent studies of alloyed ohmic contacts on III-V semiconductor materials show that in addition to the simple diffusion a formation of domains with different chemical composition occurs thereby modifying the performance of the III-V semiconductor device. The alloying is also disruptive in that it produces irregularities in the interface between the metallic doped material and the undoped semiconductor material. These irregularities can in turn result in unpredictable behavior for the semiconductor device using these contacts.
Still another technique for establishing an ohmic contact is to diffuse a dopant material from groups II, IV or VI of the periodic table of elements into the III-V semiconductor material. The amount of dopant that may be diffused has been limited by the fact that increased amounts causes a disruption in the morphology of the semiconductor surface. In the case of silicon, for example, when used as a dopant in gallium arsenide, the silicon can be doped to provide a density of about 2.times.10.sup.18 cm.sup.-3. This provides a low resistance contact but even lower resistance would be desirable.